Various types of orthopaedic devices have been described for the fixation of bone fragments. Such devices are often used to stabilize bones, thereby enhancing the healing of fractures and/or promoting union of joints being fused together. This stability helps allow early return of the function of the body part and improves patient well being.
For example, bone screws are commonly used devices for fixing bone fragments. The bone fragments are positioned in their proper configuration, and one or more holes are then drilled and tapped across the fracture. The screws are then screwed into the holes, thereby compressing the bone fragments, for example, by using a lag screw technique of over-drilling a gliding hole in the near side (cortex) of the bone. In addition, bone screws are often used in conjunction with metal plates to increase stability against multiplanar forces which may act upon the bone fragments. The screws are placed through holes in the plates, thereby lagging the plate against the bone in compression.
Metal pins (e.g. Kirschner wires and Steinman pins) are also often used to stabilize bones. The pins may be threaded or smooth, and are drilled across bone fragments to provide stability to the bone. Pins may also be used together with external fixators to gain purchase of bone fragments. These devices include an external rigid framework that lies outside of the body. Pins are inserted through the framework and into the body, by piercing the skin and inserting the pins into drilled, tapped holes in the bone.
Intrameduallary implants are another conventional device used, especially for long, tubular bone fragments. These nail-like devices are placed in the central canal of a bone to gain an interference fit and/or are locked at the ends by screws to secure the bone fragments in place. Such implants act as an internal splint within the bone, and allow compression of the bone piece during weight bearing.
Cables are also used to hold bone pieces together, for example by wrapping a cable around long, thin pieces of bone and then tensioning the cable, the bone fragments may be held together. The cables are typically held by a metal crimping piece that maintains the tension of the cable. Wrapping the cable around bone pieces in a cerlage technique is limited, however, and is generally used only to fix cracks in only one cortex of a tubular bone, or to link long slabs of bone (known as cortical struts) onto the outer surface of a tubular bone. In particular, very oblique or long spiral fractures in long tubular bones may be secured by cables to hold the bone in a stable state of compression.
Other bone anchors have been used primarily to attach suture materials to bone so that the suture may be secured to soft tissue, such as a ligament or tendon, thereby tightly linking the tissue to the bone. These devices typically anchor themselves in a hole drilled into the bone, thus preventing the device from pulling away from the bone as tension is applied to the attached soft tissue structure.
These conventional bone fixation devices generally gain their purchase to the bone by engaging the interior of the bone, i.e., the cancellous bone material. Cancellous bone is soft and spongy in nature and consequently has inferior holding power as compared with the exterior portion of the bone, i.e. cortical bone. The tension imposed on the devices after implantation may exceed the stresses that the bone-implant interface is able to resist, and may result in implant failure. Such devices may also require extensive surgical exposure of the bones to allow implantation of the devices, which may result in increased risk of infection and/or increased pain in the patient.
In contrast, external fixator devices may not require extensive surgical exposure, because they merely violate the skin with pins. However, use of such devices may be complicated by infection in and around the pin tracts beneath the skin. These external devices may also reduce patient comfort as compared to internally implanted devices.
Finally, although cable devices provide excellent tensionresistance, their use is generally limited because of the limited methods of attaching the cable to bone.
Accordingly, there is a need for an improved orthopaedic implant device that provides increased bone purchase and/or may be implanted with minimal surgical exposure.